Spark plugs and method for improving leakage characteristics thereof



Jan. 27, 1959 M. SKUNDA 2,871,389 SPARK PLUGS AND METHOD FOR IMPROVING I v LEAKAGE CHARACTERISTICS THEREOF Filed Feb. 11, 1955 llll 1% W MW H I 1 a "H my W 1 w! o M 5, x A 1. 4 ag w 3 5 a 2 W 7 1% I/l y M. \IM 11 ..\|\|w ll v/w/ "w r. W" W 2 Patented Jan. 27, 1959 ice SPARK PLUGS AND METHOD FOR IMPROVING LEAKAGE CHARACTERISTICS THEREOF Michael Skunda, Davison, Mich assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application February 11, 1955, Serial No. 487,554

4 Claims. ((11. 313-144) This invention relates to spark plugs and the method for making same and more particularly to spark plugs or" the type having an annular ground electrode crimped within the firing end of the spark plug shell.

It has been heretofore proposed to provide an annular ground electrode in spark plug bodies wherein the ground electrode was secured Within the shell by such means as soldering, brazing, or the like. Although this manner of securing the electrodes in position was found to be generally satisfactory, it was discovered that a general tendency toward loosening of the ground electrode did exist. It therefore became the practice in the art to physically secure such electrodes within the shell in addition to using brazing techniques. A common method for physically securing the electrode was to crimp the end of the shell over onto the electrode.

However, I have discovered that the application of a crimping force sulficient to properly secure the ground electrode Within the shell and enable good physical contact therebetween for good thermal and electrical conductivity resulted in a tendency for the spark plug to leak during operation despite extreme care in assembling the center electrode subassembly within the shell. Extensive investigation disclosed that the application of crimping force to the shell, supported in a die or jig about the hex, distorted the lower end of the shell along with the seating ledge provided on the inside wall surface thereof on which the center electrode subassembly was adapted to be positioned.

it is therefore an object of my invention to provide an annular type ground electrode spark plug having excellent leak-proof characteristics. It is another object of my invention to provide a simple and inexpensive method for crimping a ground electrode within a spark plug shell. It is another object of my invention to provide a method for securing an annular ground electrode within a spark plug shell without distortion of the sealing structures provided therein.

These and other objects of my invention are attained by supporting the spark plug shell between the firing end thereof and the sealing structure provided therein when applying the crimping force.

The above and further objects of my invention will be apparent from the foregoing description with reference to a preferred embodiment thereof as shown on the accompanying drawing in which:

Figure l is a partial vertical section with parts broken away of the spark plug prior to the crimping operation.

Figure 2 is a view of the lower end of the spark plug supported in a chuck and illustrating the crimping method I of my invention.

Figure 3 is a partial sectional view with portions broken away of the completely assembled spark plug.

The form of my invention shown by way of example in the drawing, is illustrated as being embodied in shielded aircraft type spark plugs, but it will be understood that my invention is applicable to other type annular ground electrode spark plugs.

Having particular reference to Figure 1, there is shown a spark ,plug 1 comprising a shell 3 which is adapted to be threadedly secured Within an engine and which is constructed to have a center electrode subassembly 5 mounted in gas-tight relationship therewith in the longitudinally extending bore 7 provided therein. A shield sleeve 9 is secured within the upper portion of the shell 3 in such manner as to maintain the subassembly 5 in sealed relationship with the shell 3. As is clearly shown in Figure 3, an annular ground electrode disk 11 is secured in the lower end of the shell 3.

The electrode subassembly 5 consists of an insulator body 13 having an intermediate raised portion 15 thereby providing an upper shoulder 17 and a lower shoulder 19 for purposes hereinafter specifically described. The insulator 13 is provided with a center bore 21 within which there is positioned a center electrode 23 having an efiicient electrical connection with a contact cap 25 adapted to be connected with an ignition cable (not shown).

The center electrode subassembly 5 is mounted upon an annular ledge 27 formed on the inner wall surface of the shell 3. A relatively soft heat resistant gasket 29 formed of any suitable material, i. e., nickel, nickel alloy,

7 'on the outer surface of the sleeve.

is positioned between the shoulder 19 and the ledge 17 in order to prevent undue localized stress in the insulator 13 and at the same time enable the insulator to be positioned on the ledge in gas-tight relationship with the shell.

In order to achieve the desired gas-tight seal, the shield sleeve 9 is formed with a bottom surface corresponding to the shape of the upper shoulder 17 formed on the insulator 13. A soft metal gasket 31 similar to gasket 29 is positioned between the lower end of the shield sleeve 9 and the shoulder 17. The sleeve 9 is made to bear upon the insulator 13 with sufficient force required for sealing by reason of the fact that the top of the shell 3 is crimped over onto a ledge 33 formed The sealing force is applied by collapsing a thin wall portion 35 provided in the shell 3 between the upper end thereof and the ledge 27. It will be noted that a braze connection 37 is also provided for in the space between the upper end of the shell 3 and the lower end of the shield sleeve 9 in order to preclude rotation of the sleeve 9 and to assure maintenance of the gas-tight seal.

The structure of the plug 1 is completed by mounting the ground electrode disk 11 in the lower end of the shell 3 and concentrically placed about the center electrode 23.

As is shown in Figure 3, the ground electrode ll is secured on a ledge 39 formed in the lower end of the shell 3 by overlapping the lower end of the shell onto the electrode disk. Inasmuch as it is necessary to obtain good electrical and thermal contact between disk 11 and the shell 3, a pressure of as much as 6000 pounds is applied to the end of the shell 41 to crimp it over the disk. The shell 3 being provided with a hex 43 on its upper portion in order to enable the plug 1 to be threadedly assembled with the engine, it was the practice in the art to support the shell at the upper end of the hex when performing the crimping operation. I have found that when the crimping pressure was thus applied, distortion occurred in that portion of the shell 3 which encloses the insulating seating shoulder and gas leakage resulted in the spark plug assembly.

I have discovered that this leakage problem may be simply and inexpensively eliminated by isolating the insulator shoulder area of the shell from the crimping pressure. This may be accomplished by supporting the shell 3 for crimping at a point between the end 41 thereof and the seating ledge 27. I have found that the simple addition of a flange to the shell in the stated position made is possible to support the crimping pressure outside the critcial sealing area of the shell, thus precluding any distortion of the sealing portions of the assembly.

As is clearly shown in, Figures 1 and 2, though the shell 3 may be supported as above described in any other suitable manner, I provide a flange 45 on the outer surface of the shell between the low-2r end 41 thereof and the seating ledge 27, the flange being small enough to be readily removed in the subsequent machining operation by which the lower end of the shell is threaded as is clearly shown in Figure 3. As is shown in Figure 2, the shell 3 may be supported about the flange 45 in a chuck or a collet 47 or other suitable holding device. I have found that a flange having as small a height as from about 0.015 or 0.018 high is. suflicient to enable the application of crimping pressures as high as 6000 pounds.

In forming the spark plug in accordance with my method as exemplified by the preferred embodiment shown on the drawing, the spark plug shell 3 is formed in the conventional manner and is provided, however, with a supporting flange 45 on its outside surface hetween the lower end thereof and the seating ledge 27 formed on its inner wall surface. The center electrode subassembly 5 is then positioned within the shell 3 with its lower seating shoulder 19 positioned onthe ledge 27, a metal gasket 29 being provided therebetween. A shield sleeve 9 having a braze material on itsouter lower surface portion is then positioned on the upper shoulder 17 of the subassembly 5, a metal gasket 31 being positioned therebetween. The upper end. of the shell 3 is then crimped over onto the shoulder 33 formed on the outer surface of the sleeve 9. The plug 1 as thus assembled is then heated in any suitable manner as by induction in order to obtain the braze connection 37 between the shell 3 and the sleeve 9,

The thus assembled plug 1 is then made leakproof by thermo-plastically collapsing a thin wall section 35 of the shell 3 or by other suitable means forcing the insulator shoulder 19 to bear tightly upon the gasket 29 and seating ledge 27. The ground electrode disk 11 is then secured upon the seating ledge 39 formed by the enlarged bore 40 in the firing end of the shell 3 and is secured thereon in concentric relation to the center electrode 23 by any suitable means such as brazing. The plug assembly 1 is then supported within a chuck or collet 47 about the flange 45 and the lower end 41 of the shell 3 is crimped over onto the ground electrode 11 by the application of a force of about 5000 pounds. The lower end of the shell is then threaded to enable the plug to be assembled within the engine, the threading operation serving to eliminate the supporting flange 45.

There is thus provided a spark plug wherein the ground electrode is secured within the firing end thereof in such manner as to enable both thermal and electrical contact between the shell and the electrode while at the same time preserving the leakproof relationship between the shell and the center electrode subassembly. The novel method by which the leakproof characteristics of the spark plug are achieved is broadly that of supporting the plug for the crimping operation at a point between the firing end thereof and the sealing structure pro vided therein, thus isolating the sealing structure from the shell distorting crimping force.

Although but a single embodiment of my invention has been shown on the drawing, it should be understood that other embodiments will be apparent to those skilledin the art and that such embodiments are within the intended scope of the claims which follow.

I claim:

1. A method for making gas-tight spark plugs of the type having an annular ground electrode secured in the firing end of the shell by overlapping engagement between the end of the shell and the electrode which includes the steps of forming a flange on the outer surface of the shell adjacent the firing end thereof, positioning an annular ground electrode inthe firing end of the shell, supporting the shell adjacent the firing end thereof and about the flange, crimping the end of the shell into overlapping engagement with the electrode, and threading the lower end of the shell to remove the flange and enable the spark plug to be secured within the engine.

2. The method of making spark plugs of the type having an annular ground electrode secured in the firing end of the shell by overlapping engagement between the end of the shell and the electrode, the shell being provided with an annular seating ledge on its inner surface and intermediate its ends, which includes the steps of forming a flange on the outer, surface of said shell between the lower end thereof and, said ledge, positioning a center electrode subassembly on the seating ledge formed within the shell and in gas-tight; relationship therewith, positioning. the ground electrode in the firing end of the shell, supporting the shell at said flange, crimping the end of the shell into overlapping engagement with the annular electrode, and thereafter threading the lower end of the shell and removing said flange, the sealing structure of the plug being thusisolated from the crimping pressure.

3. The method of making spark plugs of the type having an annular ground electrode secured in the firing end of the shell by overlapping engagement between the end of the shell and the electrode, the shell being provided with an annular seating ledge on its inner surface and intermediate its ends, which includes the steps of forming supporting means on the outer surface of the shell adjacent the firing end thereof and spaced from the seating ledge, positioning a center electrode subassembly on the seating ledge formed within the shell and in gas-tight relationship therewith, positioning the ground electrode in the firing end of the shell, supporting the shell at a position adjacent its firing end and at the supporting means, crimping the end of the shell into overlapping engagement with the annular electrode, and threading the lower end of theshell, the sealing structure of the plug being thus isolated from the crimping pressure.

4. A method of making gas tight spark plugs of the type having an angular ground electrode secured in the firing end of the shell by overlapping engagement between the end of the shell and the electrode, and includ ing an annular sealing portion on the inner surface of said shell which includes the steps of forming supporting means on the outersurface of the shell adjacent the firing end and 'spaced from the sealing portion formed on the inner surface thereof, positioning an annular ground electrode in the firing end of the shell, supporting the shell adjacent the firing end thereof and at the supporting means to isolate the sealing portion from the crimping pressure applied to the end of the shell, crimping the end of the shell-into overlapping engagement with the electrode, and subsequently threading the lower portion of said shell to remove said supporting means.

References Cited in the file of this patent UNITED STATES PATENTS 2,088,945 Xardell Aug. 3, 1937 2,235,172 Schulz Mar. 18, 1941 2,300,730 Kisbaugh Nov; 3, 1942 2,360,279 Rollert Oct. 10, 1944 2,377,481 Christie June 1, 1945 2,439,590 Tognola Apr. 13, 1948 2,664,135 Oden Dec. 29, 1953 

